Such a device for detecting positions is known for instance from DE 198 30 981 C1. The proximity sensor disclosed in this reference is used for detecting predefined positions of a movably guided part for instance of a vehicle crane. When extending the lengths of a telescope jib, for instance, the movement of the required hydraulic cylinders must be monitored and controlled, and the position of the bolts for bolting the respective lengths of the telescope jib must be detected. In the known device, the output signal of the proximity sensor is converted into digital output values by means of an A-D converter, and said output values are compared with reference thresholds, an error message being produced when the digital output values lie outside admissible reference thresholds, which is checked by an evaluation unit. In the case of a faulty conversion of the signals in the A-D converter or a faulty matching of the digital output values with reference thresholds, it may occur that no error message is produced, although the proximity switch does not operate correctly. The same is true for the case that the proximity sensor operates incorrectly in so far as independent of the position of the movably guided part always the same output signal is provided, whose digital values lie within a tolerance range.
Therefore, one object is to develop a device as mentioned above such that the functional check of the proximity switch is improved.
This object is solved by a device for detecting positions of a movably guided part by means of a proximity sensor which supplies an output signal which is a measure of the distance of an object located in front of the proximity sensor. The device can furthermore include at least one transponder arranged on the movably guided part, the proximity sensor being designed such that the data stored in the transponder can be read out by means of the proximity sensor. On the basis of the information received from the transponder it can be checked whether the proximity switch operates correctly. A functional check of the proximity switch thus can be effected, for instance, by the method described in DE 198 30 981 C1 and in addition on the basis of the communication between proximity sensor and transponder. In accordance with one aspect, a functional check of the switch thus is effected by means of two different measurement methods. This may increase the safety and degree of diagnostic coverage, whereby a higher safety class can be achieved.
The proximity sensor may be an inductive sensor which operates according to the principle of the damped LC oscillator.
Advantageously, the proximity sensor includes an oscillating circuit and a processor controlling the oscillating circuit, by means of which processor the oscillating circuit can be operated with a first frequency, at which the proximity sensor is used for detecting a position, and with a second frequency or modulated frequency, at which the proximity sensor is used for communication with the transponder. Inductive proximity sensors include a harmonic oscillator, which generates a high-frequency alternating field by means of a sensor coil which is part of the oscillating circuit. When metal enters the alternating field of the sensor coil, energy is withdrawn from the system by generating eddy current, so that the amplitude of oscillation becomes smaller. A resulting change in current is evaluated in a subsequent electronic unit. There is generated a continuous output signal, which is a measure for the distance of the object from the measurement surface of the sensor. In accordance with one embodiment, the proximity sensor furthermore is used for communication with the transponder. For this purpose, the proximity sensor can be operated at a second frequency or a frequency modulated with respect to the first frequency, or different from the first frequency. This provides for a communication between proximity sensor and transponder, this communication being used for reading out data from the transponder and furthermore, in accordance with one aspect, for writing data into the memory of the transponder.
Accordingly, it is provided in accordance with a further aspect that the transponder is both readable and writable. The transponder can be an active or passive transponder. For instance, the transponder can be designed as an inductively acting transponder.
It is particularly advantageous when the data stored in the transponder concerns the movably guided part. The data can for instance be identification numbers, serial numbers, quality data and/or data concerning the manufacturer.
A particularly advantageous application results from the fact that the data stored in the transponder may comprise data concerning the history of the movably guided part, in particular the collective loads allocated to the part. In this way it is possible to store and scan the collective loads allocated to the length of a crane jib, which is possible as a result of the allocation of the transponder(s) to a movably guided part. In the case of used devices, for instance used cranes, it is thus possible to find clues as to the collective loads allocated to the respective part by means of the data stored in the transponders, which clues form the basis for the determination of the residual value of the device.
In accordance with a further aspect, a first evaluation unit is provided, which compares the output values of the proximity sensor or values derived therefrom with reference values. Furthermore, a second evaluation unit is provided, which compares the transponder response signal with a reference signal, an error message being produced when the difference between the output values and the reference values exceeds a limit value or the transponder response signal does not correspond to the reference signal. The first and second evaluation units can be one and the same component.
In principle, it is possible that the evaluation unit constitutes a component which is suitably connected with the proximity switch. It is likewise possible that the evaluation is effected in the proximity switch itself, so that the proximity switch accordingly is designed as an intelligent component. In accordance with a further aspect, said first or second evaluation unit or also both evaluation units can accordingly be integrated in the proximity switch.
It can furthermore be provided that the values derived from the output values of the proximity sensor are digital output values, and that hence an analog-to-digital converter is provided, which converts the output values of the proximity sensor into digital values.
The proximity sensor may be connected with a control unit via a bus. It is particularly advantageous when the bus address is automatically allocated by the transponder. Furthermore, an advantageous aspect consists in that the proximity switch is automatically allocated to a mounting place on the basis of the data stored in the transponder. In this way it is possible to provide proximity sensors or switches at the desired mounting places, which at first have not yet been allocated to a certain mounting position and to which parameters have not yet been allocated correspondingly. Upon mounting the proximity switches, the associated transponder is identified, and the proximity switch is automatically allocated to the corresponding mounting place. From a central control unit, the associated parameters of the proximity switch or sensor can be read out and be allocated to the same, such as the switching distance, the switching threshold, and other functions. The same is true for the automatic allocation of the bus address by the transponder. Accordingly, it is also possible to first of all, not allocate an address to the proximity sensor and perform an automatic allocation of the bus address by means of the transponder.
It is also possible in principle that the bus address is allocated such that the transponder is scanned or read out by means of the proximity switches. In this case, the proximity switches have a default address. It is also possible and particularly advantageous to perform a direct allocation. In this case, the proximity switches have no default address, and the transponders are read out independently.